1. The Field of the Invention
This invention relates generally to audio signal processing. More specifically, the invention relates to the combining of analog and digital processing techniques to thereby capture a much wider dynamic range of an original input signal than an A/D converter can by itself, to thereby preserve a maximum amount of information from the original input signal.
2. The State of the Art
Digital audio processing includes the conversion of analog signals to digital signals, and the subsequent recording thereof. The state of the art in digital audio processing has been driven primarily by the cleaner sound that can be achieved from digitally recorded signals. The cleaner sound is an advantage resulting from the comparatively wider linear dynamic range of digital recordings.
Those skilled in the art of standard digital audio equipment understand that a typical maximum signal-to-noise specification for 16-bit digital audio systems is in the neighborhood of 90+ dB. It is understandable why this large signal-to-noise ratio is so desirable when compared to typical signal-to-noise specifications for professional analog tape. Specifically, the signal-to-noise ratio is in the neighborhood of approximately 55 dB without the aid of noise reduction. However, applying such common noise reduction systems as dbx Type I (.TM.), Type II (.TM.), or Dolby (.TM.) can achieve a signal-to-noise ratio of approximately 85 dB.
This seemingly tremendous signal-to-noise advantage of digital over analog would suggest that digital audio processing would become the unanimous and unchallenged choice for audio recording. While digital recording has substantially dominated the audio markets, the reasons extend beyond its signal-to-noise advantage. For example, digital audio recordings also provide the benefit of random access (when not recorded on tape), and the ability of the digital storage media to withstand degradation. This is in contrast to analog storage media such as tapes and LPs that can be easily wear out.
Nevertheless, despite the benefits of digital audio media, there has been a desire to return to analog equipment in the 90's. This desire is generally attributable to a unique quality and character of analog sound that is missing in digital recordings. Use of analog equipment, including relatively old vacuum tube technology, with modern digital systems has brought to light some of the previously unappreciated characteristics of analog recordings.
Those skilled in the art of recording with analog media such as tape are aware of the advantageous characteristic of being able to record high-level signals without destroying the recording. This is sometimes referred to as being able to "hit it hard." It is an unfortunate fact that the printed specifications of analog tape don't take into account the practical "headroom" available on analog media.
The maximum signal-to-noise specification of analog tape is measured by defining a "max" signal as the point at which a given signal level and frequency generate a set percentage of Total Harmonic Distortion (THD). The set percentage is typically defined as the signal level at which a 1 kHz signal produces 3% THD. However, in actual use, the signal level can easily exceed this "max" signal level by 5, 10, or even 15 dB on peaks, depending on the type of signal being recorded, and without unacceptable artifacts. Higher signal levels can be tolerated (i.e. there is more headroom) at the expense of increased THD which, incidentally, is often desirable as an effect, evidenced by the renewed popularity of vacuum tube equipment.
One conclusion is that analog recordings actually have more useable dynamic range than the technical specifications seem to indicate. For example, consider the recording of an instrument such as a kick drum. An analog tape can measure 55 dB from the 3% THD point down to the RMS (root mean square) noise floor, and the peaks of the kick drum can exceed the 3% THD level by, say, 15 dB. If this analog recording still sounds good, then there is at least 15 dB of extra and useable headroom. Accordingly, there is a total of 70 dB (55 dB plus 15 dB) of useable dynamic range. With noise reduction it is possible to easily push into the 90+ dB dynamic range territory of 16-bit digital processing. This explains why well-recorded analog master tapes make good-sounding CD's with no objectionable noise.
One main drawback of digital processing and recording is that it inherently lacks this forgiving and beneficial headroom characteristic of analog recording. Although digital conversion exhibits a wide linear dynamic range, when there is no available headroom for high-level signals, hard clipping or even ugly signal wrap-around occurs. Contributing to the bad sound produced by analog-to-digital (A/D) converter clipping is the fact that high-frequency information that is "riding" on a lower frequency waveform is completely lost for the portion of the signal which is clipped. Adding to these problems, analog-to-digital (A/D). converters exhibit their own nasty side effects such as going unstable when a modulator is overdriven by high-level signals.
Disadvantageously, these shortcomings of digital processing have drastically affected the way users operate their equipment. For example, a user who is concerned about overdriving a converter input may end up recording at lower signal levels to thereby ensure that there is ample headroom to allow for the large peaks that would ruin an otherwise perfect recording. This, of course, compromises signal-to-noise performance since the signal is now closer to the noise floor. Because users of digital equipment have to be extremely careful not to exceed 0 dB FS (full-scale) of the A/D converter, the users must use peak-reading headroom meters. In contrast, the forgiving nature of analog tape allows users of analog recording equipment the luxury of only needing to monitor an average signal level using VU meters, often having no peak indicators whatsoever.
Some audio processing equipment designers have partially addressed the problem of the unforgiving nature of analog-to-digital conversion. The designers have limited the signal being recorded by placing static limiting circuits in front of an A/D converter in order to avoid the undesirable artifacts that result from overloading.
It should be clarified that the word "static" above is used to describe a circuit that has a fixed input/output gain relationship whose gain characteristics are not dependent on a control signal. These circuits have limiting characteristics ranging from what is described as "hard", where there is a quick transition from linear to non-linear operation, to "soft", where the transition is more gradual. The more abrupt the transition, the more harsh-sounding the result. By selecting the cleaner sounding "soft" limiting, the wide linear dynamic range of digital processing is sacrificed. In contrast, selecting the harsh-sounding "hard" limiting sacrifices less of the linear dynamic range, but results in only marginal audible advantages over the clipping that otherwise occurs in the A/D converter.
Other designers or users of digital equipment have placed dynamic, as opposed to static, limiting circuits or equivalent equipment in front of the A/D converter. These dynamic circuits typically make use of a variable gain element such as a voltage-controlled amplifier (VCA), a field-effect transistor (FET), or an optically-coupled element such as a vactrol having a light-emitting diode (LED) to control the resistance of a light-dependent resistor (LDR).
These dynamic circuits or equivalent equipment have level detection circuitry which sense when the signal level exceeds a preset maximum input level that is being received at the A/D converter. In response, the level detection circuitry develops a control signal to decrease the gain through the variable gain element, thus preventing an overload condition from occurring for an extended period of time. Those skilled in the art are aware that these dynamic circuits have a finite attack time required for the detector to sense the signal level. Unfortunately, during this attack time the maximum A/D level can be exceeded, and the signal will be clipped.
To avoid this brief over-excursion into a clipping region, some designers couple static clipping circuits with dynamic limiting circuits to create "brick wall" limiting having a virtually instantaneous attack time. Those skilled in the art recognize that all these dynamic solutions add significant cost and complexity to a digital processing system. Furthermore, each has its own characteristic sound which may or may not be desirable for a particular application.
Accordingly, it would be an advantage over the state of the art to provide an analog-to-digital conversion system that could provide the benefits of the desirable audio characteristics of an analog recording, combined with the wider linear dynamic range of a digital recording.